Epidemiological studies indicate that male gametes and the complex, hormone-regulated processes by which germ cell proliferation, development and survival are controlled, may be threatened. By virtue of a long biological half-life, cadmium (Cd) exemplifies a trace environmental pollutant with potential as a cumulative toxicant. Although spermatogenesis is exquisitely sensitive to Cd toxicity, which may be related to increased permeability of the blood-testis barrier and high constitutive expression of metallothionein (Mt), attempts to pinpoint sensitive stages, cell-types and mechanisms of action have been hampered by the complex testicular organization of common laboratory mammals. A technically advantageous alternative is the shark testis, which facilitates analysis of spermatogenesis stage-by-stage in vivo and allows intact germinal units (spermatocysts), comprising stage-synchronized germ cell/Sertoli cell clones, to be isolated for experimentation and analysis in vitro. Initial studies indicate that Cd increases permeability of a functional barrier in meiotic and postmeiotic stages but is preferentially concentrated in tissues at premeiotic stages, where it increases synthesis of a Mt-like protein, increases the percentage of germinal clones undergoing programmed cell death (apoptosis), and increases secretion of SGP-2, a frequent marker of apoptosis in somatic cells. We propose to examine the hypothesis that Cd has direct access to germ cells in early developmental st ages, is taken up by an active Cd-accumulating mechanisms that is further amplified by enhanced mt expression, and activates a cell death program by perturbing normal control mechanisms. Cd-mediated defects early in development would have profound consequences for all subsequent stages and the final number of mature spermatozoa. Using the shark testis model and in vivo and in vitro approaches, we will (1) define kinetics of Cd uptake and retention stage-by-stage and identify cellular and nuclear vs. cytosolic sites of accumulation; (2) relate Cd exposure to intracellular levels and to Cd effects on barrier permeability, Mt and SGP- 2 expression, and to the extent and timing of apoptosis; (3) elucidate the relationship between apoptosis and DNA replication/repair and reproductive/hormonal status in response to Cd; (4) initiate studies to determine pathways by which Cd activates apoptosis in the stem cell/spermatogonial germ cell population; and (5) evaluate the utility of cultured premeiotic spermatocysts for identifying spermatogenic toxicants that affect programmed (apoptotic) vs. unregulated (necrotic) death of male germ cells.